1. Field of the Invention
This invention generally relates to a resistance welding process for aluminum and aluminum alloy materials, and more particularly, to a resistance welding process which enables the remarkable improvement of the electrode life in case of joining aluminum or aluminum alloy materials together by means of resistance spot welding.
2. Description of the Prior Art
A resistance spot welding process includes the steps of pressurizing an overlap portion of overlapped materials to be welded by upper and lower electrodes to apply current to the materials, then exothermally fusing a contact portion of the materials in part due to the electrical resistance of the materials to form a nugget in the contact portion, and joining the materials together.
The resistance spot welding process as described above has been often used as a joining process for rolled steel sheets in the assembly line for mass production of automobiles or the like following reasons That is, the thermal influence on the materials to be welded is small to result in small deformation of a product, the time required for welding is short to be adaptable to the mass production, and if the welding conditions are determined once, the stable and uniform joint strength can be obtained easily regardless of the skill of a welding worker or even by use of a robot.
The resistance spot welding process has the characteristics described above. However, in case of joining aluminum or aluminum alloy materials together by means of resistance spot welding, this process has such a problem as the electrode life is extremely short, in comparison with that in case of joining rolled steel sheets together by means of resistance welding.
The electrode life herein is defined as the number of times of continuous spot welding in continuously carrying out spot welding with predetermined efficiency by using electrodes which are preliminarily subjected to dressing (which means that the tip end of the electrode is cut to a predetermined shape or polished to a predetermined surface roughness) once prior to the start of welding. In general, the electrode life is evaluated on the basis of the number of times of continuous spot welding up to not more than a regular value in nugget diameter or shear strength of welded spot, or the number of times of continuous spot welding up to the occurrence of such a phenomenon as the electrode material is deposited and permanently affixed to the welded materials.
When the electrode life is evaluated on the basis of the standards as described above, the electrode life generally exceeds 10000 spot welds in the case of joining rolled steel sheets together by means of resistance spot welding, whereas the electrode life comes up to several hundred spot welds at most, in the case of joining aluminum or aluminum alloys together by means of resistance spot welding.
As the material of the electrode, use is normally made of chrome-copper alloy and chrome-zirconium-copper alloy which have satisfactory thermal conduction and high conductivity (refer to the first or second group of "Copper Electrode Materials for Resistance Welding" shown in JIS Z 3234-1977 edition). However, in case of using aluminum or aluminum alloy as the materials to be welded, it has been considered that the electrode life is shortened for the following reasons.
Namely, one of the reasons is the fact that the aluminum and aluminum alloy have far low melting points and far high thermal and electrical conductivities, in comparison with rolled steel sheet, and therefore, the aluminum and aluminum alloy have to be welded at high current and high pressure for a short period of time, in comparison with the conditions of joining the rolled steel sheets together by means of resistance welding.
Another reason is as follows: the consumption of the electrode becomes remarkable since aluminum and aluminum alloy have their surfaces covered with an insulating oxide film having a high melting point, the calorific values between the electrodes and the materials in the resistance spot welding become excessive due to the presence of the oxide film described above when bringing the electrodes into contact with the materials to apply current to the materials, and ultimately the electrodes are heated up in excess to fuse or alloy the electrodes and the materials.
For instance, in order to improve the fuel efficiency for automobiles, as material for auto body sheets, aluminum alloys have been used instead of rolled steel sheets. However, as described above, in the case of joining aluminum or aluminum alloy materials together by means of resistance spot welding, the electrode life is extremely short, in comparison with the case of joining rolled steel sheets together by means of resistance welding. Thus, it has been necessary to frequently dress the electrodes, and the resultant productivity thereby suffers. In this respect, there is a hindrance to the use of aluminum or aluminum alloy sheets as a material for automobiles.
In order to improve the electrode life in case of joining aluminum or aluminum alloy materials together by means of resistance spot welding, there has been proposed a process of welding by interposing an insert material consisting of a foil of a metal other than the aluminum or aluminum alloy between the material to be welded and each of the upper and under electrodes.
For instance, as shown in FIG. 7, Japanese Patent laid-open No. 61-159288 has disclosed a process for joining aluminum or aluminum alloys together by means of resistance spot welding by interposing insert materials 14 and 15 (e.g., Cu or Ag) having electrical conductivity higher than those of upper and under electrodes 1 and 2, respectively, between the upper and under electrodes 1 and 2 and materials to be welded 11 and 12. According to this process, even though the aluminum or aluminum alloys are welded under considerable heat input in excess, the aluminum alloys themselves are welded together without generating any surface crack and without bringing the weld penetration of the material up to the surface. This welding process can improve the electrode life to a certain degree as well.
However, when the insert materials, each consisting of Cu or Ag, are used and the materials to be welded are pressed by the electrodes at about 200.degree. C., the insert materials and aluminum are easily subjected to diffusion bonding, since Cu or Ag has good affinity with aluminum by reason of the common crystal structure (i.e., face-centered cubic lattice) shared by aluminum and Cu or Ag and the similarity in lattice constant thereof, even though Cu or Ag has high electrical conductivity and also has a melting point higher than that of aluminum.
Further, these insert materials easily become diffusion bonded to Cu in the metal structure of the electrodes. Then, the insert materials and the electrodes are fused and alloyed due to the diffusion bonding between the insert materials and the electrodes, so that the electrode life is shortened.
Furthermore, the corrosion due to the potential difference is caused by the diffusion bonding between the materials to be welded and the insert materials, and therefore, the external appearance of a product is damaged.
For instance, Japanese Utility Model Laid-open No. 3-116276 has disclosed a welding process using iron foil as insert materials in case of joining aluminum or aluminum alloys together by means of resistance welding.
However, when the same iron foil is continuously reused (e.g., continuously about two or three times) as the insert material, such iron foil insert materials gradually begin diffusion bonding to the aluminum material being welded. Then, when the iron is deposited on the materials by the diffusion bonding, the corrosion due to the potential difference between the iron and the materials to be welded is results in damage to external appearance of the product.